Cultivation information

1. temperature control

Before I go into more detail about temperature management, it is worth mentioning that the forcing date plays a decisive role, especially when it comes to temperature management. When choosing a strategy for early forcing (start of forcing in mid-November to the end of December), you should bear in mind that most varieties do not require the use of inhibitors.

This means that no strategy should be chosen that has an inhibiting effect on length growth, as this would result in too short a crop, which in turn would not generate such good yields. It should also be mentioned that strategies such as negative diff (raising the temperature during the night) for late forcing (forcing begins in mid-April) are not economically and energetically justifiable.

When it comes to temperature management, we have to differentiate whether we want to try to achieve a compact and strong structure of the plants only by lowering the general average daily temperature, or whether we want to try a so-called temperature strategy such as “Diff” or “Cool Morning”.

1.1 Influence of “Cool Morning” on the length growth of Hydrangea macrophylla

In the past, people have already considered temperature strategies to reduce the length growth of hydrangeas. This also gave rise to the idea of testing the effectiveness of “Cool Morning”. Many of the trials at that time did not achieve any significant results, although “Cool Morning” has advantages over “negative diff”, namely energy savings and year-round applicability. This in turn is necessary for late forcing in order to be able to dispense with inhibitors. (Röber and Bauer 1995)

The trial conducted by Professor Röber in Weihenstephan in 1995 was based precisely on this statement. He tested the effectiveness of “Cool Morning” on the varieties “Hermann Dienemann” (red, syn. “Leuchtfeuer”), “Nymphe” (white) and “Renate Steiniger” (blue). All three varieties rarely manage without inhibitors in forcing.

The installation date was 2 January 1995 with a temperature of 22°C from 2 January to 8 January 1995. After that, the temperature was gradually lowered by a further 2°C every 6 days until 23 January. After that, the daily average temperature remained constant at 16°C.

Sales started on 22 March 1995.

The “Cool Morning” method was carried out from 10 January to 13 March 1995. One hour before sunrise, the room temperature was lowered by 10°C for a period of four hours. Just three hours after sunrise, the original room temperature was restored.

The results were astounding:

Although no significant success had been achieved with “Cool Morning” to date, a reduction in shoot length of approx. 2.5 cm was achieved. This 2.5cm was also achieved by the inhibitor variant with a compression of 0.4% “Alar” compared to the untreated control.

On the other hand, “Cool Morning” leads to a delay in the cultivation period during the forcing phase because the daily average temperature has been reduced. This effect can be avoided by raising the temperature during the day period. (Strauch 1995)

Effects of “Cool Morning” compared to an Alar treatment in 1995 (Röber)

1.2 „Diff“

When thinking about “diff”, you should first ask yourself what strategy you are pursuing.

“Positive diff” means that the daytime temperature is significantly higher than the night-time temperature. This in turn has major energy benefits. During the day, any solar energy is utilised, while during the heating-intensive night, the temperature can be set well below the normal daytime average temperature.

These are all advantages of the “positive diff”, but unfortunately it has not yet been possible to achieve good results with hydrangeas.

The situation is different with the “negative diff”. Good results have already been achieved with this in the past. The major disadvantage of the “negative diff” is the high energy expenditure it entails. Up to now, it has had no relation to the known inhibitors. The use of “Alar”, or more recently “Desmel”, is still far more favourable in terms of costs and, in contrast to “negative diff”, can even be used all year round.

Only now are people thinking about using the more expensive options for production.

Extensive trials were carried out at the Westphalia-Lippe Horticultural Centre, “Wolbeck”, to investigate the influence of different temperature strategies on growth and plant structure.

In trials, the effects of “negative diff” were tested with the “Leuchtfeuer” variety in comparison to constant temperature control.

The “negative diff” strategy with a set temperature difference of 3°C (17°C during the day/20°C at night) led to a noticeably lower plant height compared to constant temperature control at 18.5°C. The sprouting and the cultivation period were not affected.

Influence of “negative diff” on plant growth 1993 (Richter)

Based on this result, further tests were carried out with the varieties “Leuchtfeuer” and “Libelle” at a forcing date in mid-December, in which the effects of different “positive and negative DIFF” temperature settings were compared with those of a constant temperature setting. (Figure 4).

At a “negative diff” of 8°C (15°C during the day/23°C at night), the greatest inhibition of shoot length growth was achieved compared to the constant temperature setting of 19°C.

With a “negative diff” of 4°C (17°C during the day/ 21°C at night), lower shoot length growth was also achieved.

In contrast, the “positive diff” conditions promoted shoot length growth in the corresponding plants. The different “diff” strategies had no influence on the duration of cultivation!

The “diff” strategy showed no effect on the number of shoots and cones, although a decrease in cone diameter was observed at the “negative diff” setting of 8°C.

Due to this effect on the cone diameter and thus on the quality of the plants, the “diff” setting of 17°C during the day and 21°C at night is considered optimal for cultivation.

The effect of the ?diff? strategy on the shoot length growth of hydrangeas has been demonstrated.

Parallel oxygen release and ? consumption measurements on hydrangeas of the “Leuchtfeuer” variety led to a better understanding of the mechanism of action. The measurements show that a negative oxygen balance is achieved for the daytime cycle due to high night-time temperatures in combination with low daytime temperatures.

The negative oxygen balance indicates that high night-time temperatures accelerate the respiration of those assimilates that are built up to a lesser extent during the day at low temperatures.

In addition, there are effects that indicate an altered ratio of phytohormones in the shoot and root. This is directly attributable to the higher turnover rates during the night.

The use and effect of the “Diff” strategy is particularly dependent on the external climatic conditions!

The higher the radiation from outside, the more problematic it is to maintain the low daytime temperatures! It is therefore extremely important not only to regulate temperature values, but also to check the actual temperatures in the greenhouse. The ventilation values should only be 1 to 2 °C above those of the heating. When checking the temperature, the type of sensor must be taken into account. Only ventilated and shaded temperature sensors approximately reflect the actual air temperature.

This is particularly important, as the temperature data from trials at the research institutes can usually be traced back to this type of temperature measurement. A prerequisite for the application of modern temperature strategies for growth control is therefore the modernisation of the sensors.

The effect of the “Diff” strategy can vary from farm to farm depending on the variety and the prevailing growing conditions.

The application should be used under unfavourable external growth conditions as a “positive diff” to promote line growth and as a “negative diff” to regulate growth under favourable growth conditions.

The effect of the “negative diff” is usually sufficient to sufficiently inhibit shoot length growth.

To ensure the production of high-quality hydrangeas, a combination with other alternatives for growth control, such as dry cultivation, makes sense.

The success of the “diff strategy” depends on the harmonisation of all growth factors with the goal of quality production!

Influence of different diff strategies on the length growth of plants 1993 (Richter)

1.3 Lowering the daily average temperature

If you do not necessarily want or need to be on the market with flowering hydrangeas at a very early date (February), you also have the following option of influencing shoot length growth:

Length growth can also be inhibited by starting forcing at the beginning of January and lowering the average daily temperature more quickly. The choice of variety is decisive for a compact and high-quality plant under the following cultivation conditions.

Varieties should be selected that achieve good quality under normal cultivation conditions (average daily temperature of approx. 19°C) with comparably low use of inhibitors.

In order to make the right choice of variety, you should consult the respective raw material or young plant supplier.

Normally, hydrangeas are cultivated at the beginning of the forcing period for approx. 2 weeks at 20° to 21°C until all buds have sprouted and then the temperature is lowered to approx. 18°C. To achieve a more compact plant, you can also start with approx. 18°C and lower the average daily temperature to approx. 15° to 16°C after 2 weeks. The result is a more compact plant with harder shoots, firmer cones and a more intense colour. The reduction that this method entails is not very great. In addition, you have to reckon with a longer cultivation time of about 1 to 2 weeks.

This variant is not a panacea either, but if it is used in conjunction with the timely cultivation measures, for example, the compression effect is also increased.